1. Field of the Invention
The present invention relates to a process for separating amino acids, an amino-acid-based monomer, and a process for the preparation thereof, as well as a polymer material, and a process for the preparation thereof.
2. Description of the Related Art
Amino acids are structural elements of one of the most important types of molecules in nature, viz. proteins. It is most essential that amino acids of high purity are available, inter alia, as the starting material in different syntheses and as components in infusions.
Moreover, most amino acids are chiral, and with few exceptions only one enantiomer is of interest. The access to efficient purification methods for amino acids is essential in order to prepare such compounds of high, especially optical, purity.
Today a number of fundamentally different processes are available for purifying amino acids with regard to both the type of amino acid and the enantiomeric form. These processes may be divided into three main types: chemical, chromatographic and enzymatic. However, only a few of them are suitable for nonderivatized amino acids which are mainly of interest in industrial applications, since derivatization of such relatively inexpensive fine chemicals is unrealistic from the economic point of view.
The present invention relates to a new, preferably chromatographic, process of purifying nonderivatized amino acids.
The molecular imprinting technique implies preparing a synthetic polymer of desired affinity for a certain molecule (the so-called print molecule) (See B. Ekberg and K. Mosbach, Trends in Biotechnology, Vol. 7, 92-96, 1989; G. Wulff, American Chemical Society Symp. Ser., Vol. 308, 186-230, 1986). The polymer is composed of monomer units and intermediate cross-linking agents. The network contains a print of the print molecule. The preparation can be divided into three main steps, as shown in FIG. 1. First, there are formed chemical interactions between the monomers (of the same or different chemical structures) and the print molecule in a selected solvent. These interactions may be both noncovalent and covalent. After adding cross-linking agents and an initiator, there is formed in the second step a polymer round the complex of monomer and print molecule. This results in a "mould" round the print molecule. The last step comprises extraction for removing the print molecule from the polymer, whereby an affinity seat remains in the polymer. This affinity seat is a molecular print of the print molecule in the polymer.
The molecular print means that a form of adsorbtive "memory" for the print molecule has been formed in the polymer. This implies that the print molecule will be better adsorbed to the polymer than a molecule which is structurally related to the print molecule (see FIG. 2). Depending on the type of print molecule, a substrate- or enantio-selective polymer is obtained. This can then be used for specific separation of the print molecule in a subsequent separation process, e.g. liquid chromatography. In the extraction step, the print molecule can be recovered.
Fundamentally, the functional monomer and also the cross-linking agent are selected according to the interactions that are desired between this and the print molecule. The interactions can be either noncovalent or (reversibly) covalent. The covalent bonding is not as general as the noncovalent. Furthermore, more drastic chemical conditions must be applied to remove the print molecule during the extraction step. When noncovalent bondings exist, significantly milder extraction conditions can be applied. The noncovalent bonding is the most general type of bonding, since there are more interactions between the print molecule and monomers at the same time as a mixture of different monomers can be used. (See L. I. Andersson, "Molecular recognition in synthetic polymers; A study of the preparation and use of molecularly imprinted polymers, thesis, Applied Biochemistry, University of Lund, 1990).
The molecular imprinting technique has been applied to prepare polymers having the above described selectively adsorptive properties for amino acid derivatives (but not nonderivatised amino acids which are the subject matter of this invention) and smaller peptides (See B. Ekberg and K. Mosbach; G. Wulff; L. I. Andersson; D. O'Shannessy, B. Ekberg and K. Mosbach, J. of Anal. Biochem., Vol. 470, 391-399, 1989; L. I. Andersson and K. Mosbach, J. of Chrom., Vol. 516, 313-322, 1990), .beta.-blockers (See L. Fischer, R. M uller, B. Ekberg and K. Mosbach, J. Am. Chem. Soc., Vol. 113, 9358-9360, 1991), carbohydrate derivatives and carbohydrates (See G. Wulff), ketones (See K. J. Shea and T. K. Dougherty, J. Am. Chem., Soc., Vol. 108, 1091-1093, 1986), etc.